Peroxisomes Notes

Peroxisomes

• Not part of the endomembrane system because they are not derived from the ER.

• Proteins in the cytoplasm pull membrane lipids out of the ER and stick them in the peroxisome to form it.

• Peroxisomes can divide to create more of them.

• Prominent in liver and kidney cells.

• Contain oxidative reactions that form hydrogen peroxide ($H<em>2O</em>2$).

• Contain catalase to degrade hydrogen peroxide.

Functions of Peroxisomes

Detoxification of Harmful Substances

• Catalase uses toxic substances like methanol or ethanol as electron donors, making them less toxic.

• Detoxifies reactive oxygen species such as superoxide and hydroxyl radicals using superoxide dismutase.

  • These free radicals are generated during normal cellular metabolism.

Oxidation of Fatty Acids

• 25-50% of fatty acid oxidation occurs in the peroxisomes.

• Important for degrading:

  • Long chain fatty acids (16x to 22x).

  • Very long chain fatty acids (24x to 26x).

  • Branched fatty acids.

• Once fatty acids are shorter than 16 carbons, they are sent to the mitochondria to be fully broken down.

Metabolism of Nitrogen Compounds

• Most animals (not primates) use urate oxidase to oxidize urate.

  • Urate forms during catabolism of nucleic acids and some proteins.

  • Urate oxidase breaks down urate into allantoin and hydrogen peroxide.

• Primates produce urea and uric acid.

  • Urea is created from the breakdown of proteins.

  • Uric acid is created from the breakdown of nucleic acids.

• Allantoin is broken down into allantoic acid or urea and excreted.

• Birds secrete uric acid (the white part of their poop) because they don't urinate.

Synthesis and Degradation of Amino Acids

• Peroxisomes have aminotransferases that play a role in synthesizing and degrading amino acids.

Catabolism of Unusual Substances

• Some oxidase substrates are rare compounds that the cell can't degrade otherwise.

• Degrade D-form amino acids (L-form is used to make proteins).

• Degrade xenobiotics (chemical compounds foreign to biological organisms), such as alkanes found in oil.

Disorders of Peroxisomes

• Most common disorder: X-linked adrenoleukodystrophy.

  • Defective protein is an integral membrane protein responsible for transporting very long chain fatty acids into peroxisomes for oxidation.

  • Buildup of very long chain fatty acids destroys the myelin sheath, causing problems with neurons.

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